1. Field of Invention
The invention relates to a method for measuring the electromotive force constant of motors, and more particularly to a method for measuring the electromotive force constant when motors work in single phase
2. Related Art
Three phases permanent magnet motors, which are also called Permanent Magnet Synchronous Motors or DC brushless motors, are dominant in the industry because of their superior control and response. The most common examples are servo motors used in the automation industry, or spindle motors of disk drives to hard disks used in the Office Automation (OA) field.
The general three-phase permanent magnet motors are in Y-connection structure, which can be categorized in three-wired types and four-wired types according to wires of motors. Three-wired type motors have a three-phased winding to be connected with motor drivers. The servo motors used in factory automation belong to this category. Four-wired type motors have three phase windings and a neutral winding. The small permanent magnet motors used in the Office Automation (OA) field belong to this category.
In the magnetic parameters of permanent magnet motors, the electromotive force constant Kemax, which is equal to the torque constant in M.K.S., is closely linked to the motor performance, driving force and operation. The prior art discloses some solutions for measuring the electromotive force constant.
One of the solutions is an off-line anti-electromotive force approach, which utilizes a servo controllable motor to connect with a to-be-tested motor. The to-be-tested motor is open, i.e., is not connected with any drivers. Once the motor rotates in constant electrical angle velocity ωr, the electromotive force of the to-be-tested motor is obtained through the electromotive force, induced by any two phases.
However, the approach has some technical problems. For example, an expensive controllable motor and drivers are necessary. A clip fixture is also needed for coupling the two motors without slant. If the two motors slant too serous, the servo motor may not rotate smoothly in constant velocity because the load and the bearing of the to-be-tested motor is also easily damaged. Furthermore, some spindle motors employing sir bearings for hard disks lose the air characteristic after coupling with another motor. Therefore, such kinds of motors are not suitable for this approach.
The other solution is the on-line vector control estimation approach. The reference coordinates for servo permanent magnet motors often adopt a rotator coordinates system. Therefore, when the input current of the q axis is set to be constant and the input current of the d axis is set to be 0, the motor rotates in fixed torque. After current control performed by two close-loop controllers until the loop is in steady state, the back electromotive force constant is obtained accordingly.
However, the above approach is time consuming and cost wasting because of the servomotors. Furthermore, when applying the approach to small motors, additional circuits are needed, and a precise encoder attached on the motor is also needed for implementation. Meanwhile, in this method, the estimation error of the current and the resistor, and the design of controllers affect the measured electromotive force constant Kemax.
R.O.C patent publication No. 488125 discloses a method for identifying the magnetization of rotators through the electromotive force constant. Some auxiliary windings are wound on the stator core for sensing the magnetic flux of the magnetic field of the rotator. The electromotive force constant is obtained through the electromotive force induced on the auxiliary windings.
However, this method is only suitable for the magnetization of rotators when manufacturing motors. The auxiliary windings are especially subscribed and the motor has to be driven in constant velocity and close loop. This method is not suitable for finished motors because the stators can not be refit. Furthermore, if the motors do not have a velocity sensor, the close loop control is not provided for constant rotating.
The electromotive force constant Kemax affects the motor performance, driving force, and operation. However, the prior art did not provide effective solutions for this technical problem. Therefore, a method for measuring the electromotive force constant is necessary for motor technology.